206 MOVEMENTS AND EXCHANGES OF FLUIDS 



277, Comparative Amount of Transpiration of Stems and Leaves. 

 The preparation used in the last experiment should be set aside, and 

 on the following day should be weighed at the same hour on which 

 the previous experiment was begun, and again 8 or 10 hours 

 later. Note the total loss of weight and compare with that of 

 previous test. Deduct from total of previous day and obtain cor- 

 rected rate. Compute rate per square centimeter of surface of 

 stem. A small plant of sunflower will be found suitable for this 

 experiment. When the leaves are taken off, it should be done 



FIG. 104. Recording balance, with pot of earth on one pan and plant on the other. 

 The difference of loss of weight of the two is traced by a pen on the cylinder, moved 

 by clockwork beneath., 



by cutting the petioles neatly near their bases, and applying gela- 

 tine to the excised surfaces. Guttation if present will invalidate 

 the results. 



278. Influence of Light on Transpiration. Make a preparation 

 as in 276 and obtain the rate of transpiration during a period 

 of four hours in bright sunlight, using a thermograph and hygro- 

 graph, to make a continuous temperature and humidity record. 

 Now set the plant in a dark room and keep similar records dur- 

 ing an equal period. If the temperature and moisture are equal, 



